Modern aircraft are equipped with a generous assortment of sensors and instrumentation to facilitate operation of the aircraft. Nearly every parameter critical to safe and efficient operation is accurately monitored in real time, with one notable exception. The gross weight and longitudinal center of gravity, both critical to safe and efficient operation, are based on a variety of inputs, which include a mixture of predetermined “known” values, measured values, and estimates based on statistical sampling. The Load and Trim Sheet, or Manifest, is the outcome of the load planning process that utilizes these inputs. Although the load planning process is highly controlled, there are opportunities for undetected errors. In addition, even if the load plan is perfect, the actual loading process can introduce errors that cannot be detected without a real time measurement of the “as loaded condition” of the aircraft. Furthermore, during the loading process the aft section can become disproportionately heavy (due to early aft loading of cargo and/or baggage) resulting in the aircraft tipping (tail down), which can cause damage to the fuselage.
According to the National Aerospace Laboratory Report Number NLR-TP-2007-153, the majority (more than 90%) of weight and balance problems identified could be eliminated if there was a system available to the flight crew that would do an automatic onboard weight and balance assessment.
Present on-board aircraft weight and balance systems (WBS) that measure strain in the landing gear structure, require a multitude of precision sensors (usually one or more per wheel location) historically resulting in a system that is expensive and impractical for smaller lower cost aircraft. In addition, these systems require features in the landing, or special “add-on” adapters to facilitate the attachment of the sensors to the landing gear structure, making retrofit of these systems either expensive or impractical. Therefore, only the very high value large wide body aircraft currently utilize these systems.
There are also several prior art versions of on-board WBS that utilize strut pressure measurements to determine the weight supported by each “leg” (strut) of the aircraft. This is an appealing concept because it only requires one pressure sensor for each landing gear strut (the load is ideally just the pressure times the piston area) and these sensors can be attached to existing servicing ports. However, these systems were historically inaccurate due to the high static friction or “stiction” associated with the seals between the moving strut piston and the outer cylinder that contains the strut gas or liquid (or both). To overcome this static friction a number of creative methods have been employed. These include a system of pumps and valves and/or heaters and valves used to modulate the pressure within the strut until the static friction is overcome. Using “smart algorithms” these approaches have been shown to significantly improve the accuracy of the system, however, they also tend to add weight and complexity to the system as well as inducing additional complex failure mechanisms.
Another potential approach to on-board WBS is to measure the tire pressure and calculate the change in the weight support by each tire as a function of the change in the tire pressure. This approach is appealing because tire pressure measurement systems are already in use on many aircraft (though not for this purpose). There are two fundamental problems with this approach; first of all, the current tire pressure measurement systems are not precise enough; and secondly, tire pressure can vary greatly between tires and with differing environmental conditions for the same applied load (weight supported by the tire). With regard to measurement precision, because tire pressure is only a secondary function of the load supported by the tire, it only changes about 5% for a fully loaded aircraft (most of the change in load results in a change in the tire footprint area and not a change in the tire pressure). For this reason the pressure measurement must be very sensitive (5% of pressure equals 100% of load, so 1% of load equals approximately 0.05% of pressure). For large aircraft tires this equates to a measurement accuracy of approximately 0.1 PSIG. This is achievable with current precision pressure sensors, but not those typically used for aircraft tire pressure only. Therefore, improved accuracy tire pressure sensors would generally be required.
With regard to environmental variables, the difference in normal tire servicing pressures (+/−5 PSIG) is almost equal to the full range of pressure change resulting from loading, and tire pressure can vary greatly over time due to temperature changes and small undetected leaks. In addition to these factors, the pressure per pound of load is also influenced by the tire characteristics (i.e., tread and sidewall stiffness, etc), and these can vary over the life of the tire. Although some of these variables can be characterized or monitored, the residual variations associated with using tire pressure to determine the load support by the tire are quite large (roughly the same magnitude as the total change in pressure due to loading), and tend to making this approach impractical if not impossible.